Heretofore, from the viewpoint of high production efficiency, curing energy cost reduction and VOC reduction, an active energy ray curable system has been widely applied. Above all, a UV-curable system is the mainstream since the facility introduction cost is low and the installation area can be small as compared with any other active ray curable system.
The photopolymerization initiator that is the essential component in the UV-curable system remains in a cured product as it is therein the photopolymerization itself or as a decomposed matter thereof, differing from a reactive monomer that is to form a polymer form after curing to be fixed in a cured film. Almost all photopolymerization initiators that are distributed in the art are low-molecular-weight compounds, and therefore the remaining photopolymerization initiator itself or decomposed matter thereof is also a low-molecular-weight substance, and heretofore this is a cause of an offensive odor.
Further, recently, it has become pointed out that the remaining material may migrate toward the substance that is kept in contact with the cured product, and in particular, in a UV-curable ink for use in food-wrapping prints, the remaining material may migrate toward the back surface of the print that is in direct contact with food, and the regulation on migration of a photopolymerization initiator is being severer year by year.
For this, there has been made a trial of reducing migration by making a photopolymerization initiator have a reactive group to fix it in a cured film.
For example, in PTL 1 and PTL 2, there are disclosed oligomer-type photopolymerization initiators having plural initiator groups in the molecule. The method disclosed in these publications is effective for reducing odor and migration by oligomerizing the photopolymerization initiator. However, against the recent regulations, the effect is still insufficient.
PTL 3 discloses a UV-curable resin obtained through Michael addition reaction of a photopolymerization initiator having an α-aminoacetophenone structure and a polyfunctional acrylate. According to the method disclosed in PTL 3, a reactive group is introduced into the photopolymerization initiator group so as to fix the photopolymerization initiator group in a cured coating film to thereby greatly reduce the odor of the photopolymerization initiator in the cured film. However, the photopolymerization initiator disclosed in PTL 3 is somewhat poor in photopolymerization performance and especially in use that requires a curing rate such as in a UV-curable ink or the like, there occurs a problem of curing insufficiency.